The present invention relates to a superheterodyne receiver.
FIG. 1 shows in block diagram form a conventional superheterodyne receiver. In such a superheterodyne receiver, a received RF signal is supplied to a first mixer 2 through an aperiodic RF amplifier 1 and mixed with the output of a variable-frequency local oscillator 3. A second mixer 5 is connected to the first mixer 2 through a BPF (Band Pass Filter) 4. The RF signal at the output of the BPF 4 is mixed with the output of a fixed-frequency local oscillator 6 and thus converted to an IF (Intermediate-Frequency) signal. An IF amplifier and detector 8 are connected to the second mixer 5 through a BPF 7, whereby a low-frequency (baseband) output is obtained at the output of the IF amplifier and detector 8. An AGC (Auto Gain Control) amplifier 9, connected to the output of the IF amplifier and detector 8 and provided with an AGC voltage corresponding to the output level of the IF amplifier and detector 8, supplies an AGC voltage to the IF gain control input terminal of the IF amplifier and detector 8. A level detecting circuit 10 is connected to the output of the RF amplifier 1 so as to obtain a level control voltage corresponding to the RF output level of the RF amplifier 1, which level control voltage is supplied to the gain control input terminal of the RF amplifier 1.
In the receiver thus constructed, the RF signal received from an antenna 13 is amplified by the RF amplifier 1. As shown in FIG. 2, the level detecting circuit 10 has a flat characteristic. Consequently, the amplification factor of the RF amplifier 1 is controlled directly by the output signal level of the RF amplifier 1, specifically, the amplification factor is controlled so that the higher the output RF signal level of the RF amplifier 1, the smaller the amplification factor of the RF amplifier 1. The output RF signal of the RF amplifier 1 is subjected to frequency conversion. Namely, the output RF signal of the mixer 2, which is at a first intermediate frequency f.sub.1 higher than the receiving RF band, is filtered by the BPF 4. The output of the BPF 4 is supplied to the mixer 5 for conversion to a second intermediate frequency lower than the first intermediate frequency. The output RF signal of the mixer 5 is filtered by the BPF 7 before being supplied to the IF amplifier and detector 8, whereby a detected signal is produced by the IF amplifier and detector 8. The AGC voltage corresponding to the output signal level of the IF amplifier and detector 8 is applied from the AGC amplifier 9 to the IF amplifier and detector 8, whereby the IF amplification factor is controlled.
In this conventional superheterodyne receiver, because the RF amplifier 1 is aperiodic and has a flat amplification characteristic in the receiving band, the tuning frequency is determined by the mixer 2, the variable-frequency local oscillator 3 and the BPF 4. However, if an interference signal sufficiently stronger than the desired signal is present in a frequency range other than the tuning frequency range in the receiving band, the gain of the RF amplifier will be controlled by the level detecting circuit according to that strong interference signal. Consequently, the receiving sensitivity is suppressed, making reception difficult.